A prior-art thermal printhead is disclosed for example in JP-A-8-258309. The disclosed thermal printhead includes a heat sink plate, a head substrate, and a circuit board. The head substrate and the circuit board are bonded to the heat sink plate with an adhesive. The head substrate has an upper surface on which a heating resistor and a plurality of drive ICs are mounted. The circuit board carries a connector for external connection and has an upper surface provided with a wiring pattern for connecting the drive ICs to the connector.
The thermal printhead further includes a protective cover formed of a synthetic resin. The protective cover is so provided as to cover the wiring pattern on the circuit board and the drive ICs on the head substrate. The protective cover has a lower surface which is integrally formed with a plurality of engagement pawls. By flexing the protective cover widthwise appropriately, the engagement pawls are brought into engagement with the circuit board and the head substrate, thereby mounting the protective cover onto the thermal printhead. Conversely, the protective cover may be removed from the thermal printhead by disengaging the engagement pawls from the circuit board and the head substrate.
The prior art thermal printhead has the following problems. As described above, since the protective cover mounted onto the circuit board and the head substrate is flexed, it tends to restore to its natural state, exerting a force on the circuit board and the head substrate to separate them from the heat sink plate. As a result, the circuit board and the head substrate may be partially lifted from the heat sink plate.
Further, mounting the protective cover in such a flexed state onto the thermal printhead may cause another problem. In printing, a printing paper and/or an ink ribbon is pressed against the heating resistor on the head substrate by a platen roller. At that time, the protective cover which is undesirably flexed widthwise comes into contact with the printing paper and/or the ink ribbon strongly at a particular portion or portions. As a result, the printing paper and/or the ink ribbon may be broken or wrinkled.